I've had exactly one very successful free promotional day.
The bottom line about the KDP Select free days are...the free day in and of itself is absolutely nothing more than a tool. There is no magic in it. Your book absolutely will not jump in the rankings simply because you schedule a free day.
In fact, just as you would normally promote your rear end off when your book is selling for a monetary figure, you'll also need to promote your rear off for the free promotional day or days that you schedule.
"But I thought you said you've had one very successful promo day," you ask.
Yes, you're right. And here's the difference.
Ok, for the promo day I just had, I gave away approximately 150 copies....TOTAL in the US of my book. My ranking peaked at 25 from what I saw. And, during this day, my book was tweeted by myself and a few others through out the day. It was also facebooked by myself and a few others through out the day. Plus it was promoted by Flurries for Words (or something like that), Author Marketing Club or one of their lists, and a couple of other venues. In other words, my book was promoted by a few different venues and several individuals. And that resulted in about 150 copies in the US. Yes, I had some in the UK as well, and 1 copy in each of a couple of other countries. Seriously, one copy. But, I'm focusing on the US numbers because that's normally our bread and butter.
Now, the very successful day I had, I literally had a couple of thousand downloads of my book before I even remembered that it was free that day.
What was the difference?
It kinda makes me sick to say this, because it makes me sick to admit that we authors are dependent on one or two different sites/venues to "make us" successful. But, here's the rotten truth. That one day, my book was mentioned by Ereader News Today.
This round, my book was mentioned by three or four other sites along with friends and myself pimping it all day. for a grand total of 150 copies in the US. When Ereader News Today (ENT) promoted it, it downloaded around 3,000 copies total in one day without me saying a word.
In fact, from what I've gathered, if ENT or Pixel of Ink promote your book, you can actually expect around 3,000 copies to be downloaded in one day.
When this many copies are downloaded in one day, this shoots your book up to the top or near the top of the chart in your genre. My book, All Jacked Up, stayed at the number one spot for a day or two when ENT mentioned my book.
P.S. There is absolutely nothing wrong with any of the other venues that you can use for book promotion on your free days. In fact, I think they are doing a great thing...AND I wish they were much bigger than they are and have much more impact than they do. I hate it that the only sites with a real impact are Pixel and ENT. In fact, if you'll look at this very website of mine, I have a section on this for free books. I'm trying to build this site up to have the kind of reach that Pixel and ENT have, and hopefully help more authors get their names out there. So please, check out my page on this site as well.
Back to my original point. Here's the benefit of KDP Select. Once your free day is over, or days, your book automatically goes back to being charged for. If you go "old school" as in through Smashwords and that process to get Amazon to go free, you will have a harder time getting your book back to paid. With Select it automatically goes back to paid.
Other than that, KDP Select does absolutely nothing in and of itself for your book as far as promotional value.